Oxyalkylene polymers represented by an oxymethylene polymer comprised mainly of repeating --CH.sub.2 O-- units are well known. In this regard, the processes for the preparation of oxyalkylene polymers according to the prior art can be broadly classified into two types. One main type of prior art process involves polymerizing anhydrous formaldehyde as a principal monomer, while the other main type of prior art process involves polymerizing, as a principal monomer, a cyclic acetal such as trioxane (which is a cyclic trimer of formaldehyde).
It has been proposed with respect to the former process to polymerize or copolymerize substantially anhydrous formaldehyde in the presence of an anionically or cationically active catalyst. On the other hand, it has been proposed with respect to the latter process to polymerize or copolymerize a cyclic acetal (such as trioxane) in the presence of a cationically active catalyst. The catalyst which has been proposed for the above processes includes Lewis acids, particularly, halides of boron, tin, titanium, phosphorus, arsenic or antimony, such as boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride and complexes and salts thereof; proton acids such as perchloric acid; esters of proton acids, particularly, esters of perchloric acid with lower aliphatic alcohols, such as t-butyl perchlorate; anhydrides of proton acids, particularly, mixed anhydrides of perchloric acid with lower aliphatic carboxylic acids such as acetyl perchlorate; trimethyloxonium hexafluorophosphate, triphenylmethyl hexafluoroarsenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate and acetyl hexafluoroarsenate. Among them, boron fluoride and coordination compounds thereof with organic compounds such as ethers are the most preferred catalysts for the polymerization or copolymerization of a cyclic oligomer of formaldehyde, such as trioxane, and are typically used on an industrial basis. However, polymer prepared by using any of the above noted catalysts has a limited degree of polymerization. Therefore, it is very difficult to prepare a polymer having a degree of polymerization exceeding a certain upper limit according to the prior art.
Furthermore, when a cyclic ether or cyclic formal having two or more adjacent carbon atoms is copolymerized with a principal monomer as described above (i.e., so as to introduce a relatively more stable unit into the polymer's chain) copolymer obtained just after the copolymerization generally has a thermally unstable moiety on its terminal. Thus the copolymer must be stabilized by eliminating the unstable moiety prior to its practical use, thereby necessitating complicated and uneconomical post-treatment. If the content of the unstable moiety in crude acetal copolymer directly obtained by copolymerization is reduced, the final product will thus be more stable and the post-treatment will advantageously be simplified. Thus, a copolymerization process for preparing an acetal copolymer containing a reduced amount of unstable moiety formed during the polymerization process has been desired.
The reason for the above problems with respect to the limitation in the degree of polymerization of an acetal (co)polymer and the presence of a considerable amount of an unstable terminal moiety in acetal copolymer is presumably because the polymerization catalyst as described above not only accelerates the polymerization, but also affects the decomposition and depolymerization. That is, the molecular weight, thermal stability, processability in molding and the color of acetal (co)polymer vary depending upon the kind of the catalyst used. In this regard what has been needed in the art is an acetal polymerized process which uses a polymerization catalyst which overcomes the above problems. It is towards attaining such a process that the present invention is directed.